Combustion engines having a wide variety of designs and fueling strategies have been known for many years. The use of different fuels in an engine depending upon fuel availability, combustion characteristics, and engine operating environment has become commonplace. Jurisdictional requirements may also suggest different fuel types or fuel blends for different seasons. The use of winter diesel versus summer diesel in compression ignition diesel engines is a well-known example.
In certain engines, a given control strategy may be implemented regardless of variation in fuel type or fuel blend characteristics. This may be the case because the engine is relatively insensitive to fuel type/quality variation, or because the engine is not equipped with certain control hardware to address such variation. In other engines, particularly internal combustion engines employing modern engine control and emissions reduction strategies, the manner in which the engine or related systems are controlled may depend upon the type of fuel being used. For instance, an electronically controlled engine might calculate fuel injector control signal duration based on one map for a first fuel type but based on another map for a second fuel type. In still other instances, the frequency or manner in which exhaust particulate filters are regenerated or controlled might vary depending upon what type of fuel is being used in an engine. In addition to fuel type, the quality of a particular fuel being used, such as the relative amount of impurities like water, might also affect what engine control or emissions reduction strategy is chosen. To optimally make use of available engine controls and engine subsystems, it will be readily apparent that identification of the fuel type or fuel quality being used in an engine may be required.
For the reasons explained above, internal combustion engines may employ various mechanisms for determining the type or quality of fuel being used therein. Sensors adapted to interact with fuel or combustion products have been proposed. Such sensors tend to add expense and complexity to engine systems, however. In recent years, engineers have also proposed ways to test fuel type or quality by observing operation of an engine under certain conditions without directly sensing fuel type or quality. One example of a strategy for observing engine behavior and indirectly deducing fuel properties is known from U.S. Pat. No. 5,817,923 to Ohsaki et al. (hereinafter “Ohsaki”). Ohsaki proposes to measure a time period between turning on a starter switch or initiating fuel injection and the engine reaching a predefined rotation speed. The duration of the time period, an engine speed gradient during the time period and a change amount in rotation during the time period are determined. By weighting these factors, a controller purportedly determines whether the fuel in use is heavy or light. Ohsaki may have certain applications, but the necessary calculations are relatively complex and, moreover, a multiplicity of confounding factors may exist during engine start-up which could compromise the integrity of the strategy under field conditions.